Scaling of transistor gates to smaller dimensions requires processing of thinner gate stack side-wall insulating layers. Such insulating layers, referred to herein as gate spacer layers, are typically composed of silicon nitride. These layers are typically applied conformally to the substrate and then patterned using subtractive wet or dry etch processes. The process must be highly selective to the substrate material and etch the insulating layer anisotropically. These requirements make dry etching the preferred method since wet etching is typically isotropic.
U.S. Pat. No. 4,529,476 to Kawamoto et al. disclose a dry etching gas suitable for selective etching of silicon nitride and a process for selectively dry-etching silicon nitride with the dry-etching gas consisting of C, H, and F atom species and having a ratio of F to H by atom of not more than 2.
Reyes-Betanzo et al. disclose higher oxide and nitride film roughness results for oxygen-free plasma etch processes using CF4 or SF6 etch gases (Vac. Sci. Technol. A 17 (6) 3179 (1999)).
U.S. Pat. No. 6,117,791 to Ko et al. and WO2002/03439 to Micron disclose the use of fluoroethane to selectively etch doped SiO from undoped SiO and SiN. Thus, these applications promote SiN as an etch stop for fluoroethane.
US2011/0068086 to Suzuki et al. and US2013/105916 to Chang et al. disclose anisotropic silicon nitride etch processes providing selectivity to silicon and silicon oxide using fluorohydrocarbon gases having the composition CxHyFz, wherein x is an integer selected from 3, 4, 5, and 6; y and z are positive integers; and y is greater than z.
It is well known in the art that SiN/SiO2 and SiN/Si selectivity increases as the ratio of H to F increases (i.e. CH3F>CH2F2>CHF3). See, e.g., Chen et. al. (Microelectronic Engineering 86, (2009)).
However, reports have shown that the preferred etch gas, CH3F, may implant carbon into the silicon substrate during nitride spacer etching, requiring an additional processing step (HBr/O2 or N2/H2 plasma) to remove C—Si bonds prior to the silicon epitaxy step. Blanc et al., Journal of Vacuum Science & Technology B 32 (2) (March/April 2014).
Additionally, as further explained in more detail in the present application, while H rich etch molecules may selectively etch silicon nitride, some of these etch gases have characteristics which are not desirable for application in gate spacer layer etching.
A need remains for SiN etching processes suitable for gate spacer layer etching without detriment to the processes that follow.